1. Field of the Invention
The present invention relates to a voltage regulator that outputs a constant voltage, and more particularly, to an overcurrent protection circuit that reduces an output current to protect a circuit when an overcurrent flows into an output terminal.
2. Description of the Related Art
Voltage regulators are employed as voltage supply sources of circuits in various electronic devices. A function of the voltage regulator is to output a constant voltage to an output terminal without being affected by a voltage variation of an input terminal. Moreover, it is important that the voltage regulator has a function of overcurrent protection in which an output current is reduced to protect a circuit when a current that is supplied to a load from the output terminal increases and exceeds a rated current by a predetermined value (for example, refer to JP 02-189608 A).
FIG. 5 illustrates a circuit diagram of a voltage regulator including an overcurrent protection circuit. The conventional voltage regulator including the overcurrent protection circuit includes an output voltage divider circuit 2 that divides a voltage of an output terminal Vout, a reference voltage circuit 3 that outputs a reference voltage, an error amplifier 4 that compares the divided voltage with the reference voltage, an output transistor 1 that is controlled by an output voltage of the error amplifier 4, and an overcurrent protection circuit 50. The overcurrent protection circuit 50 includes an output current detection transistor 5 and a detection resistor 6 which form an output current detection circuit connected in parallel to the output transistor 1, and a transistor 7, a resistor 8, and an output current control transistor 9 which form an output current limiting circuit that is controlled by a voltage of the detection resistor 6.
The overcurrent protection circuit 50 as described above has a function of protecting a circuit from an overcurrent through the following operation.
In the case where an output current of the output terminal Vout increases, a detection current in accordance with the output current flows through the output current detection transistor 5. The detection current flows through the detection resistor 6, thereby allowing a voltage between a gate and a source of the transistor 7 to increase. In this case, the overcurrent flows into the output terminal Vout, and the voltage between the gate and the source of the transistor 7 exceeds a threshold voltage, whereby a drain current flows through the transistor 7. When the drain current of the transistor 7 flows through the resistor 8, a voltage between a gate and a source of the output current control transistor 9 decreases. Accordingly, a drain current flows through the output current control transistor 9, thereby allowing a voltage between a gate and a source of the output transistor 1 to increase. The overcurrent protection circuit 50 operates in this manner, and accordingly the output current of the output terminal Vout is suppressed to have fold-back drooping current-voltage characteristics.
FIG. 6 illustrates a graph of the fold-back drooping current-voltage characteristics. In the current-voltage characteristics, a value of the output current on which the overcurrent protection circuit acts is called a limit current. Further, a value of the output current when the output terminal Vout is short-circuited and the output voltage is equal to a ground potential is called a short-circuit current.
However, in the conventional overcurrent protection circuit 50, accuracy of a current value of the limit current decreases due to a process variation that is generated when the transistor 7 is manufactured. In addition, accuracy of the short-circuit current decreases due to a variation generated in the resistor 6. However, it is difficult to adjust the transistor 7 and the resistor 6 with accuracy when they are manufactured.
For this reason, there occurs a problem that, when the limit current is set to be small, variations in short-circuit current causes deterioration of starting characteristics of the voltage regulator because of a relationship between the output current and the output voltage. In other words, it is difficult to sufficiently reduce the limit current for securing the starting characteristics of the voltage regulator.
Further, an internal temperature of the voltage regulator increases as a result of effects of heat generation caused by the overcurrent, an ambient temperature, and the like. However, in the conventional overcurrent protection circuit 50, it is impossible to control the limit current value and the short-circuit current value by the internal temperature of the voltage regulator.